Magnetic flowmeters having a general tubular shape are placed as a section in a pipeline allowing fluid to pass through them. Two metal electrodes are placed diametricaly opposite each other in the tube wall, to contact an electrically conductive fluid. An external magnetic circuit is used to generate a magnetic field across the diameter of the pipe and at right angles to the axis of the electrodes. A voltage is then induced by the conductive fluid flowing across the magnetic field according to Faraday's law. The induced voltage is detected by the electrodes and is proportional to the fluid flow rate.
To prevent the measurement voltage from leaking off, insulating material is placed around the field measurement zone. If the tube is metal, it is lined with an insulating material such as polytetrafluoroethylene. Alternatively, the tube may be made wholly from a non-magnetic electric insulator, such as a ceramic. Lined tubes have limited erosion resistance, relatively poor mechanical stability, and problems with loosening and shifting of the liner. In either case the electrodes pass through holes in the tube wall and liner, if any, to contact the fluid. A light-tight seal must be made between the electrode and the tube wall.
Ceramics are now being used as flow tubes. High purity aluminum oxide ceramic is an excellent flow tube material because of its excellent corrosion and erosion resistance. Alumina is a high electrical insulator and has good mechanical stability and strength. These material features allow industrial magnetic flowmeters to be used with a wide variety of liquids including very strong acids and bases; and at both high temperatures and pressures. As a result, and in particular due to the high material quality provided by ceramic flow tubes, high material standards have been placed on the electrode and seal portion of the meter. Platinum is recognized as the best electrode material for the chemical, electrical, and mechanical demands. It functions well but is, however, expensive. The remaining problem is then to seal the electrode to the ceramic given the harsh operating conditions.
Platinum electrodes are currently fused in ceramic flow tubes. A platinum wire is placed through the wall of the ceramic tube while the ceramic is still in an unfired or "green" state. The tube and wire are then kiln fired at about 1750.degree. C. The ceramic shrinks around the wire in what is hoped to be a leak-tight seal. Achieving a leak-tight seal is difficult because platinum has a slightly higher thermal expansion coefficient than ceramic and therefore shrinks away from the ceramic as the assembly cools from the kiln firing temperature. Pressure tests have proven fused joints to be unreliable for leak rates of 10.sup.-8 atm-cc/sec of helium or less. Differential contraction then makes the integrity of the fused seal questionable from the time of manufacture. Also, due to the thermal expansion mismatch and thermal cycling, incomplete fusing, scratches in the metal wire, and chemical attack of the fused joint, the fused joint is subject to failure in service.
Another problem with fusing the electrode to the ceramic is repair. Once the tube leaks the costly ceramic is lost, and the valuable platinum fused in the ceramic is recovered only with difficulty. Poor or erratic production yields along with the inability to repair the seal or salvage either the ceramic tube or the platinum electrode makes the fused-in electrode an expensive production process.
Traditional compressed washer type seals are felt to be unreliable. Since the ceramic surface on a microscopic level is rough, there are small packages between the ceramic and the washer that allow helium passage during testing. Over time it is felt, such passages would also leak corrosives leading to failure. Traditional O-ring seals made of soft, resilient materials such as silicone, fluorocarbon or nitrile rubbers might seal the micropassages, but such softer materials are felt to lack durability, and in particular to lack the chemical resistance required. One O-ring material that may work is a terpolymer, of perfluoro(methylvinyl ether) and tetrafluoroethylene and a perfluorinated cure site monomer. It has completely fluorinated bonds making it chemically suitable. Unfortunately the material is expensive, and may lose its sealing properties under cold conditions.
Fluoroplastics such as polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), and perfluoroalkoxyethylene (PFA) have long been accepted in magnetic flowmeter applications as having the best chemical and temperature withstanding capability. They are used as liner materials for metal flow tubes. It has been found that a spring-loaded fluoroplastic gasket placed between the electrode head and the counterbore in the ceramic does not provide the leak tightness required. This is due both to the rough surface texture of the counterbore and the inability of the fairly rigid gasket to flow into the rough ceramic surface and seal off the small passages. Lapping of the counterbore inside the tube may smooth the surface for a tight seal, but takes time and is subject to error and is therefore not considered attractive from a production standpoint.
It is therefore an object of the invention to provide a seal to a ceramic material. An additional object is to provide a seal able to withstand pressure, heat and corrosion resistance to a broad range of chemicals from strong acids to strong bases. Still another objective is to provide a seal allowing repair and recovery of costly components.